The oxychlorination of ethylene is a well-known, industrially employed process that is carried out in fluidized-bed and fixed-bed reactors. Despite the more uniform temperature distribution in the reaction zone the disadvantages of the fluidized-bed process must not be overlooked, such as back-mixing and abrasion problems as well as certain difficulties in the flow behavior of the catalyst, which can lead to an agglomeration of the catalyst particles, whereby in particular the reaction selectivity is adversely affected. For example, the technological advantage of an oxygen cycled gas procedure with up to 80 vol. % of ethylene in the cycled gas, in which the released enthalpy of reaction is optimally dissipated corresponding to the present state of the best available technology, cannot be employed in a fluidized-bed oxychlorination.
On the other hand in the fixed-bed process, in which moreover the reactants can also be more accurately controlled, the oxygen cycled gas procedure can be carried out satisfactorily with a high ethylene content in the cycled gas. The suppression of the disadvantages of the fixed-bed process, such as in particular the occurrence of local temperature peaks (so-called hot spots), increased pressure drop over the reactors as well as a gradual decomposition of the shaped supported catalyst due to coke inclusions and thermally conditioned long-term effects that adversely affect the activity and selectivity as well as the catalyst life, has been the subject of numerous investigations and proposed improvements that have been disclosed in the literature, such as the addition of activity-enhancing and selectivity-enhancing promoters, the addition of thermal stabilizers to the support material, the selection of flow-promoting and/or thermodynamically and reaction kinetically advantageous catalyst geometries and porosities and technical improvements by adopting the so-called multireactor technology by splitting up the hydrogen chloride and/or air and/or oxygen into the individual reactors connected in series, with and without cycling of the gas.
The following printed specifications have been named as part of the particular prior art:
Allen, J. A., Clark, A. J., Rev. Pure and Appl. Chem. 21, 148 (1971)=D1 PA1 DE-A-17 68 453=D2; PA1 Allen, J. A., Clark A. J., J. Appl. Chem., 26, 327 (1966)=D3; PA1 DE-A-20 50 061=D4; PA1 U.S. Pat. No. 4,446,249=D5; PA1 FR-2021986=D6; PA1 Dotson, R. L., J. Catalysis 33, 210 (1974)=D7; PA1 Villadsen, J., Livbjerg, H., Catal. Rev. Sci. Eng. 17 (2), 203 (1978)=D8; PA1 Shatchortswa, G. A. et al., Kinet, i Katal. 11, 1224 (1970)=D9; PA1 Ruthren, D. M., Kennedy, C. N., J. Inorg. Nucl. Chem 30, 931 (1965)=D10; PA1 Dirksen, F., Chemie-Technik 12 (1983) No. 6, 36-43=D11; PA1 EP-A-0 582 165=D12; PA1 EP-A-0 775 522=D13; and PA1 WO 96/40431=D14. PA1 a) 0.5-15 wt. % of one or more Cu-II compounds, the quantitative amounts referring to copper metal, PA1 b) 0.1-8 wt. % of one or more alkali metal compounds, the quantitative amounts referring to alkali metal, PA1 c) 0.1-10 wt. % of an oxide mixture comprising PA1 d) the remainder up to 100 wt. % being .gamma.- and/or .alpha.-aluminum oxide as support material, wherein PA1 e) the support material d) has a total pore volume in the range from 0.65 to 1.2 cm.sup.3 /g, and wherein PA1 f) the supported catalyst is present in the form of cylindrical hollow bodies having at least one passage channel, the ratio of height h to external diameter d.sub.e being less than 1.5 for diameters d.sub.e of up to 6 mm, and the ratio h/d.sub.e being less than 0.6 for diameter d.sub.e greater than 6 mm. PA1 low pressure losses; PA1 low bulk densities; PA1 relatively large external surfaces per unit volume of a reactor vessel; PA1 uniform flow through the passage channel or channels; PA1 generation of high reaction gas turbulences in the interior of the catalyst moldings; PA1 generation of high reaction gas turbulences around the shaped catalyst bodies; PA1 increase in the axial and radial mixing, in other words mass transport against the flow direction under the influence of a concentration gradient; PA1 enhancement and facilitation of the diffusion capacity of the gaseous reactants in the channels; PA1 enhancement and facilitation of the gaseous reactants in the catalyst pores as a result of mesopore dominance; PA1 high turbulences of the reaction gases at the catalyst moldings and in the reaction space ameliorate the disadvantageous heat dissipation problems that occur on account of reduced heat exchange coefficients between the catalyst moldings and reaction gases; PA1 achievement of higher space-time yields; PA1 increase in conversion; PA1 increase in selectivity; PA1 increase in catalyst service life. PA1 i) support materials d) having the geometry f) are charged with soluble precursors of the components c1) and c2); PA1 ii) the precursor compounds are converted into the oxide form, and PA1 iii) the oxidically charged support materials are loaded with the components a) and b). PA1 iv) shape a mixture of the components c) and d) into the geometry f), and PA1 v) load the shaped support materials with the components a) and b).
D1 discloses a large number of promoters and activators, such as the oxides and/or chlorides of lanthanum, platinum, zirconium, uranium, cerium, thorium, titanium, tantalum, rhodium, molybdenum, ruthenium, tungsten, europium as well as didymium mixtures of rare earth metals, which in some cases are said to favorably influence the pure Deacon reaction as well as the oxychlorination reaction.
D2 describes a fluidized-bed oxychlorination catalyst based on silica as support, which in addition to copper chloride and alkali metal chloride contains, besides chlorides of the rare earths or chlorides of scandium, zirconium, thorium and uranium, also yttrium chloride. Although yttrium-III chloride reacts with oxygen with the release of chlorine and formation of Y.sub.2 O.sub.3, a gas-impermeable oxide film is formed however (D3). The system YCl.sub.3 /Y.sub.2 O.sub.3 is thus not very suitable for the fixed-bed catalyst since on account of the film-like oxide coating the catalyst surface agglomerates and the rate of reaction between YCl.sub.3 and oxygen very rapidly decreases. In the fluidized-bed process on the other hand there is a mechanical destruction of the film-like oxide coating due to the constant mutual friction of the catalyst particles.
D4 claims a supported catalyst for the fluidized-bed or fixed-bed oxychlorination, which in addition to copper chloride also contains compounds of the rare earth metals with atomic No. 62 and above and/or compounds of yttrium on active aluminum oxide as support material. In contradiction to D2, the presence of alkali metals is deleterious since they reduce the activity of the catalysts.
According to D5 and D6 additives of compounds of the rare earth metals, in particular compounds of lanthanum, are used in the fluidized-bed process in order to prevent an agglomeration of the fluidized-bed catalyst particles.
The same effect of the lanthanide metal compounds is described by R. L. Dotson in D7.
According to D8 however lanthanum-III chloride increases the sublimation rate of copper chloride/potassium chloride systems, as a result of which such catalysts lose their activity over time.
In D9 it is assumed that lanthanum, on account of its strong complex-forming tendency, removes chloride ions from the copper chloride/potassium chloride system, whereupon the potassium/copper chlorocomplex becomes depleted in chloride and thus the copper chloride volatility is increased.
According to D10 the activating effect of lanthanum-III chloride is based only on an increase in the pre-exponential frequency factor in the Arrhenius equation, without reducing the activation energy, since it accelerates the velocity-determining re-oxidation of the copper-I species that is formed as intermediate. Since this frequency factor is however considerably less dependent on temperature than the reaction velocity constant k, the catalytic effect of lanthanum-III chloride is overall weaker.
In order to improve the temperature stability and thermal resistance of an oxychlorination catalyst based on gamma-aluminum oxide as support, i.e. in order to increase its resistance to elevated temperatures during its useful life without altering the physical structure of the transitional alumina matrix, there are used inter alia additives of lanthanum oxide or thorium oxide (D11) and other oxides of the rare earth metals (D4), though of course it is the hydrated aluminum oxide, which is used to obtain the activated aluminum oxide, that is of decisive importance for the temperature stability of the resultant porous solid. The gamma-aluminum oxide obtained by boehmite dehydration is most stable as regards heat treatment, in which connection during the action of elevated temperatures under hydrothermal conditions of an oxychlorination reaction there is a gradual conversion during the use of the gamma-aluminum oxide to the catalytically inactive but thermodynamically stable alpha-aluminum oxide, presumably after partial rehydration to boehmite and diaspore (equilibrium) which already at 420.degree. C. transforms into alpha-aluminum oxide, whereas by thermal decomposition of gamma-aluminum oxide alpha-aluminum oxide is formed only at 1000.degree. C. The gamma-aluminum oxide that crystallizes in the tetragonal spinel form is converted into hexagonal rhombohedral alpha-aluminum oxide. This change in the physical structure and morphology leads to a gradual disintegration of the shaped catalyst particles, whereby the pressure drop can increase over the reactor packing right up to the bed compaction, and on account of the different disintegration rates the gas distribution over the reactor cross-section becomes increasingly less uniform. Both these effects result in the catalyst having to be replaced prematurely.
In addition it is known that the mechanical strength of supported catalysts depends on their geometry and porosity, in which connection it is fairly generally accepted that thick-wall support geometries with large diameters and supports with a relatively small pore volume are mechanically most stable.
D12 discloses a catalyst and a process for the oxychlorination of ethylene to dichloroethane; the catalyst comprises a support that has an active metal composition comprising 2 to 8 wt. % of copper as chloride, or in the form of other copper salts, 0.2 to 2 wt. % of an alkali metal (alkali metals), 0.1 to 9 wt. % of a rare earth metal (rare earth metals) and 0.05 to 4 wt. % of a metal (metals) of Group IIA of the Periodic System of the Elements (IUPAC 1970), all percentages by weight being referred to the total weight of the catalyst composition, wherein all metals are deposited on the support and the catalyst composition has a specific surface in the range from 20 to 220 m.sup.2 /g. The support is aluminum oxide with a bulk density in the range from 0.8 to 1.1 g/cm.sup.3 and a pore volume of between 0.2 and 0.5 ml/g.
D13 discloses catalysts for the oxychlorination of ethylene in the form of hollow cylindrical granules having at least three passage channels. A production process is disclosed in which catalysts are formed having a fairly large porosity and a fairly narrow pore radius distribution. The porosity is in the range from 0.2 to 0.5 cm.sup.3 /g, while the BET surface is in the range from 80 to 380 m.sup.2 /g. D14 is similar, and discloses car wheel-shaped catalysts for the oxychlorination.
It has however been shown that the previously specified instructions and assumptions, which to some extent are contradictory, are overall incapable of achieving fully all the demands placed on supported catalysts.